Pulsing system for electrostatic accelerator



A. J. GALE 2,967,258

PULSING SYSTEM FOR ELECTROSTATIC ACCELERATOR Jan. 3., 1961 Filed Aug. 19, 1957 CHA PG/A/G cuRREA/r PULSING SYSTEM FQR ELECTROSTATIC ACCELERATOR Alfred J. Gale, Lexington, Mass., assignor to High Voltage Engineering Corporation, Burlington, Mass, a corporation of Massachusetts Filed Aug. 19, 1957, Ser- No. 678,856

3 Claims. (Cl. 315-3) This invention relates to electrostatic accelerators and in particular to a novel system for pulsing the charged particle beam which is accelerated by such accelerators.

The principle of the belt-type electrostatic accelerator is well known, and such accelerators are disclosed, for example, in United States Patent No. 1,991,236 to Van de Graaif and No. 2,252,668 to Trump and at vol. XI, page 1 of Reports on Progress in Physics (1948). An endless belt, usually of insulating material, conveys electric charge to a hollow electrode where the accumulation of charge raises the potential of the hollow electrode to high voltage. This high voltage may then be used to accelerate charged particles to high energy within an evacuated acceleration tube one end of which is electrically connected to the hollow electrode and the other end of which is grounded.

It has been difficult to pulse the output of the acceleration tubes of electrostatic accelerators because of the fact that the potential of the hollow electrode will fall unless the current carried thereto by the belt during the pulse is sutficient to compensate for the increased current leaving the hollow electrode as a result of the pulse of charged particles in the acceleration tube. The pulsing system of the invention provides. a simple means by which an adequate charging current is assured during the pulse.

It will be clear from the following description that the pulsing system of the invention is equally applicable to the pulsing of positive or negative charged particle beams. Therefore, while in the following detailed description the invention is described with particular reference to an electron accelerator, it is to be understood that the invention also includes the pulsing of other charged particle beams such as positive ion beams.

The invention may best be understood from the following detailed description thereof having reference to the accompanying drawing in which:

Fig. l is a highly diagrammatic view of an electrostatic accelerator showing an electric circuit embodying the invention; and

Fig. 2 is a graph showing certain current relationships in the apparatus of Fig. 1.

Referring to the drawing, and first to Fig. 1 thereof, negative electric charge is sprayed onto an endless belt 1 of insulating material in the conventional manner from spray points 2. The belt 1 carries this negative electric charge to the inside of a hollow electrode 3, where the negative charge is collected by collector point 4 which are connected to the hollow electrode 3 through a resistor 5. The collector points 4 are also connected to the grid 6 of a triode 7 which is supported within the hollow electrode 3 and which serves as a voltage amplifier. The cathode 8 of the triode 7 is connected directly to the hollow electrode 3 and the plate 9 of the triode 7 is connected to the cathode cup 10 of the main acceleration tube 11. The main acceleration tube 11 is connected in the usual manner, the cathode end of the main acceleration tube 11 being connected to the hollow electrode 3 and the anode end thereof being grounded, so

Fatent 7 margin Patented Jan. 3, 1961 that electrons which are allowed to pass through the cathode cup 10 are accelerated to ground through the main acceleration tube 11 by the electric field which is produced in the main acceleration tube 11 by the accumulation of negative electric charge on the hollow electrode 3. Plate 9 is also connected to a plate voltage source 12 through a resistor 13. As is well known to those skilled in the art, the main acceleration tube 11 is essentially a triode, and the cathode cup 10 serves the function of a grid, while the cathode filament 14 serves the function of a source of charged particles and the anode 15 serves the function of a plate or target to wards which these charged particles are accelerated. The power supply 16 which feeds current to the spray points 2 is adjusted to give a square wave output varying be" tween a negative current 1' and i as shown in the graph of Fig. 2.

Assume now that the negative current of 1' has been sprayed onto the belt 1 for some time, so that the negative current flowing from the belt 1 through the resistor 5 is the same as the current through the column resistor 17. The column resistor 17 is conventionally used in electrostatic accelerators and usually comprises a multiplicity of resistors connected in serie between the hollow electrode and ground. As long as the belt carries electric charge to the hollow electrode, the potential of the hollow electrode will continue to rise until the sum of all the currents therefrom to ground equal the charging current carried thereto by the belt. In order to provide controlled opera tion, the column resistor 17 or some similar device is necessary in order to provide a controlled leakage path. Because the current through the resistor 5 is very small, the grid 6 is only slightly negative with respect to the cathode 8, so that the triode 7 conducts and current flows through the resistor 13, with the result that the cathode cup 14) is biased negative to cut-off and there is no beam current in the acceleration tube 11.

When the higher negative current 1' reaches the collector points 4 the resultant negative current flowing through the resistor 5 biases the triode 7 to cut-off, so that no current flows through the triode 7 and the resistor 13. As a result, the potential of the cathode cup 10 goes positive, becoming that of the plate supply 12, and the acceleration tube 11 conducts during this portion of the cycle. This portion of the cycle, of course, constitutes the pulse and it will be appreciated that during the pulse the belt 1 is carrying more current to the hollow electrode 3 than during the intervals between pulses. If this were not so, the charging current carried by the belt 1 would be insur..- cient to support the pulse without an accompanying voltage drop at the hollow electrode 3 during the pulse, with a corresponding voltage rise at the hollow electrode 3 between pulses. In accordance with the invention, the current through the column resistor 17 and the voltage of the hollow electrode 3 are substantially constant throughout the cycle. When the current collected at the collector points 4 returns to 1' the main acceleration tube 11 returns to cut-off. It will be appreciated that the effect on the main acceleration tube 11 occurs when the current changes on the belt I reach the collector points 4 and not when the current changes are initiated at the spray points 2.

In essence then the invention comprehends a pulsing system for electrostatic accelerators wherein there is provided a pulsed charging current which is synchronized with the pulsed charged particle beam so that throughout the cycle the voltage of the electrostatic accelerator remains substantially constant.

Having thus described the principle of the invention together with an illustrative embodiment thereof, it is to be understood that although specific terms are employed they are used in a generic and descriptive sense and not for purposes of limitation, the scope of the invention being set forth in the following claims.

I claim:

1. Apparatus for producing a pulsed beam of-charged particles comprising, in combination with an electrostatic accelerator including a hollow electrode, a charge conveyor for carrying electric charge to said hollow electrode, means for delivering a charging current to said charge conveyor, a charge collector for collecting electric current from said charge conveyor at said hollow electrode, and an acceleration tube having a source of charged particles, means for accelerating charged particles from said source as a beam, and a grid member adapted, by virtue of its potential, to control the beam current issuing from said source through said acceleration tube: means for causing said charging current to alternate between a higher and a lower value; a resistive path electrically connected between said charge collector and said hollow electrode, whereby the potential of said charge collector is a function of the collected current; and means for causing changes in potential of said charge collector to produce corresponding changes in the potential of said grid member such that said grid member alternately transmits and cuts off the beam current in said acceleration tube as the current collected by said charge collector alternates between a higher and a lower value, respectively, in response to the alternation in said charging current.

2. Apparatus for producing a pulsed beam of charged particles comprising in combination with an electrostatic accelerator including a hollow electrode, a charge conveyor for carrying electric charge to said hollow electrode, means for delivering a charging current to said charge conveyor, a charge collector for collecting electric current from said charge conveyor at said hollow electrode, and an acceleration tube having a source of charged particles, means for accelerating charged particles from said source as a beam, and a grid member adapted, by virtue of its potential, to control the beam current issuing from said source through said acceleration tube: means for causing said charging current to alternate between a higher and a lower value; a resistive path electrically connected between said charge collector and said hollow electrode whereby the potential of said charge collector is a function of the collected current; means for amplifying the potential difference across said resistive path; and means for applying said amplified potential difference between said source of charged particles and said grid member in such a manner that said grid member alternately transmits and cuts off the beam current in said acceleration tube as the current collected by said charge collector alternates between a higher and a lower value, respectively, in response to the alternation in said charging current.

3. Apparatus for producing a pulsed beam of electrons comprising in combination with an electrostatic accelerator including a hollow electrode, a charge conveyor for carrying negative electric charge to said hollow electrode, means for delivering a negative charging current to said charge conveyor, a negative charge collector for collecting negative electric current from said charge conveyor at said hollow electrode, and an electron acceleration tube having a source of charged particles, means for accelerating charged particles from said source as a beam, and a grid member adapted, by virtue of its potential, to control the beam current issuing from said source through said acceleration tube: means for causing said negative charging current to alternate between a higher and a lower value; a resistive path electrically connected between said charge collector and said hollow electrode, whereby the potential of said charge collector is a function of the collected current; a triode having a cathode, grid'and plate connected to said hollow electrode, said charge collector, and said grid member, respectively; and a plate voltage supply connected to said plate through a resistive path, whereby changes in potential of said charge collector produce corresponding changes in the potential of said grid member such that said grid member alternately transmits and cuts off the electron beam current in said electron acceleration tube as the negative current collected by said charge collector alternates between a higher and a lower value, respectively, in response to the alternation in said charging current.

References Cited in the file of this patent UNITED STATES PATENTS 2,058,732 Simon Oct. 27, 1936 

